In manpage, it says the output will be named as `<output-filename>1.page`, so when output-filename is `x.page`, the output should be `x.page1.page` instead of `x1.page` <-- which is the current behaviour

My opinion is 
- If no `output-filename` is specified, use `<input-filename-without-suffix>%d.page`
- Otherwise there must be a valid placeholder in the specified template.
- If a users specifies a output-filename without a placeholder, we may either use the old behaviour (appending `%d.page`) or show a warning about the change here.

What do you think?